1. Field of the Invention
The present invention relates to an adjustable fitting ring structure. In particular, this invention relates to an adjustable fitting ring structure that is applied to a safety helmet and can be adjusted according to the wearer's head circumference.
2. Description of the Related Art
In order to protect themselves when they are driving a bicycle, riders need to wear safety helmets. Because safety helmets are not manufactured in a personalized manner, an adjustable fitting ring is added to the safety helmet. Therefore, the wearer can adjust the fitting ring to match their head circumference.
Reference is made to FIGS. 1 and 2. The adjustable fitting ring structure of the prior art includes a fitting ring 1a, a connecting base 2a, a guiding part 3a, and an adjustable part 4a. The fitting ring 1a has two end portions 11a. The upper side and the lower side of the two end portions 11a both have tooth portions 12a. 
The connecting base 2a includes a first connecting part 21a and a second connecting part 22a that can connect together. The two side surfaces of the connecting base 2a each have an opening 23a. The two end portions 11a of the fitting ring 1a respectively pass through the two opening 23a. The tooth portions 12a are received in the connecting base 2a. A positioning convex block 211a protrudes from the connecting surface between the first connecting part 21a and the second connecting part 22a. 
The guiding part 3a has a gear 31a. The gear 31a is connected with the tooth portion 12a of the fitting ring 1a. The guiding part 3a assembles with the first connecting part 21a. 
The adjustable part 4a is connected with the guiding part 3a to rotate the guiding part 3a. The adjustable part 4a includes a gear disk portion 41a, a middle portion 42a, and a connecting portion 43a. These portions are assembled together in order. A plurality of cone gears 411a is disposed on the gear disk portion 41a in a radial direction. The cone gears 411a are connected with the convex block 211a. 
However, the end portions 11a of the fitting ring 1a of the adjustable fitting ring structure of the prior art are geared with the upper side or the lower side of the gear 31a via the gear portion 12a. Therefore, the end portions 11a easily slip out and escape from the gear 31a. Unattended to, the gear 31a will not rotate smoothly.
Furthermore, the adjustable fitting ring structure of the prior art uses the cone gear 411a of the adjustable part 4a to contact the positioning block 21a of the connecting base 2a to position the adjustable part 4a and prevent the adjustable part 4a from rotating automatically. The adjustable part 4a makes the guiding part 3a rotate to change the dimension of the fitting ring 1a. After the adjustable fitting ring structure has been used for a long time, the cone gear 411a and the positioning block becomes worn, and the user feels uncomfortable when he or she adjusts the adjustable fitting ring structure.
Moreover, the quantity of elements required for the adjustable fitting ring structure of the prior art is large. The assembly process is complex, and the manufacturing costs are subsequently high.